There are abundant reserves of low-grade coals (low-quality coals) which are porous and which contain much moisture such as lignite, sub-bituminous coal, and the like. Since such low-quality coal has a low heating value per unit weight and has poor transport efficiency, the low-quality coal is heated and dried to increase the heating value per unit weight, and is compression-molded to improve the handling capability.
Meanwhile, the above-described low-quality coal which has been subjected to the heating process is likely to react with water to form a hydrate. In addition, the low-quality coal is likely to react with oxygen in the air because it has an increased surface activity after generation of radicals or the like caused by release of the carboxyl group or the like from the surface. This leads to a possibility of spontaneous ignition due to a reaction heat generated in the reactions.
For this reason, Patent Literature 1 below and the like propose production of reformed coal in which the spontaneous ignition is suppressed in the following manner. Specifically, low-quality coal is added to mixture oil obtained by mixing heavy oil with solvent oil, followed by heating (100° C. to 250° C.), so that the moisture in the coal is vaporized from pores thereof. In addition, after the mixture oil enters the pores, the solid part is separated from the liquid part and then dried. Thereby, the entire surface of the solid part, including the pores therein, is coated with the mixture oil to be shielded from the outside air.